Apparatus for crucible-free zone melting of semiconductor material



SepL'G, 1966 w. KELLER 3,271,115

APPARATUS FOR CRUCIBLE-FREE ZONE MELTING OFSEMICONDUCTOR MATERIAL 1 Filed June 21, 1965 2 Sheets-Sheet 1 Sept. 6, 1966 KELLER 3,271,115

APPARATUS FOR C IBLEFREE Z E MELTING OF SEMICONDUCTOR MATER Filed June 21, 1965 2 Sheets-meet 2 United States Patent 3,271,115 APPARATUS FOR CRUCIBLE-FREE ZONE MELT- ING OF SEMICONDUCTOR MATERHAL Wolfgang Keller, Pretzfeld, Germany, assignor to Siemens-Schuckertwerke Aktiengesellschaft, Erlangen, Germany, a corporation of Germany Filed June 21, 1965, Ser. No. 469,048 Claims priority, application Germany, June 20, 1964, S 91,638; Mar. 29, 1963, S 84,442 7 Claims. (Cl. 23--273) My invention relates to an apparatus for crucible-free or floating zone melting of semiconductor material.

This is a continuation-in-part of my application Serial No. 354,329, filed March 24, 1964.

The apparatus constructed in accordance with my invention is of the type which employs a vertically mounted rod-shaped semiconductor element and an induction heating coil displaceable along the axis of the rod for producing a melting zone, as vwell as a subsidiary induction heating coil for preheating or after-heating the solid semiconductor material. An apparatus of the aforementioned type is described, for example, in the German patent DAS 1,007,885.

In known apparatuses of this type, subsidiary induction heating coils or a few coil turns are located above and beneath the induction heating coil that surrounds the semiconductor rod and produces the melting zone. These additional heating coils also surround the semiconductor rod and are spaced slightly therefrom. As a result of the slight spacing between the separate coil turns and of the variation in the spacing of the turns from the semiconductor rod, a slight heating effect is exerted on the latter. The temperature gradient from the melting zone into the solid semiconductor material is thereby able to be kept very small so as to avoid disturbances or dislocations of the crystal.

A disadvantage of the known apparatuses of this type is that during the relative displacement of the melting coil and the semiconductor rod that is rigidly clamped at both ends, the end clamps of the semiconductor rod cannot be moved up to the melting coil, but only to the preheating and after-heating coils. In this way the entire clamped length of the semiconductor rod cannot be subjected to the zone melting but only a relatively short portion thereof. This results in an uneconomical use of the zone-melting equipment as Well as of the semiconductor rod that is being processed.

It is accordingly an object of my invention to provide an apparatus for the crucible-free zone melting of semiconductor material which overcomes the aforementioned disadvantages of the existing apparatuses and which affords a solution for the problems encountered in those known apparatuses.

To this end and in accordance with a feature of my invention, I provide in an apparatus for crucible-free zone melting of a rod-shaped semiconductor member, a subsidiary induction heating coil having longitudinally extending turns, portions of which are substantially parallel to one another. The subsidiary induction heating coil is located in such a position that these portions of the turns extend in a direction parallel to the axis of the rod-shaped semiconductor member.

In accordance with another feature of my invention, the portions of the coil turns arranged substantially parallel to one another are located in the peripheral surface of an imaginary cylinder and respectively alternately connected with one another at the ends of the imaginary cylindrical surface so that a meandering or sinuous conductive line is formed.

The novel features which are considered as characteristic for the invention, are set forth in particular in the appended claims.

While the invention has been illustrated and described as embodied in apparatus for crucible-free zone melting of semiconductor material, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. Such adaptations should and are intended to be comprehended within the meaning and range of equivalents of the claims.

The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of a specific embodiment when read in connection with the accompanying drawings, in which:

FIGS. 1 and la are perspective views of two different embodiments of the zone-melting apparatus of my invention; and

FIG. 2 is a transverse sectional View of the embodiment of FIG. 1a taken at a location above the heating coil 5.

Refer-ring now first to FIG. 1 of the drawing, there is shown a semiconductor rod 2 clamped at both ends in two clamping holders 3 and 4. The semiconductor rod ends are set into two ceramic tubes 3a and 4a which are in turn securely held in both clamping holders by suitably tightening the associated set-screws. The semiconductor rod 2 is heated by a single turn induction heating coil 5 that is connected to an electric power source (not shown) which produces a melting zone 6. In accordance with my invention, there is also provided an additional or subsidiary induction heating coil 7 which has an elongated shape and which consists of coil turn portions that extend parallel to one another. The induction coil 7 is disposed in such a manner that the longitudinal axis of the semiconductor rod 2 lies parallel to the turn portions, so that the heating effect of the coil 7 extends over substantially the entire length of the semiconductor rod. The metal components of the clamping holders 3 and 4 such as the set-screws, for example, are suitably located beyond the heating effect of the coil 7.

One of the rod clamping holders can be made rotatable in a manner well known in the art. Relative movement between the semiconductor rod 2 and the melting coil 5 occurs, for example, by displacing the rod clamping holders 3, 4 while the coils 5 and 7 are held stationary. It is, however, preferable that the semiconductor rod be held stationary by its clamping holders and only the melting coil 5 be displaced vertically as shown in the figure. In such a case the subsidiary heating coil 7 can also be fixed with respect to the semiconductor rod, which provides a further advantage of the apparatus constructed in accordance with my invention in that only one relatively small component of the apparatus, namely the melting coil 5, has to be moved. All of the components of the apparatus can be compactly installed in a vacuum chamber. The apparatus of my invention can, however, also be operated under protective gas.

The induction coil 7 for preheating the semiconductor rod can for example be 30 centimeters long and as shown in the drawing can consist of two windings or turns. The portions of the windings extending parallel to each other are spaced apart a distance of 70 millimeters for example. A supplied heating capacity of about 2 to 3 kilowatts is suificient for heating a silicon rod of about 25 mm. diameter with a specific resistance of about 1,000 ohm centimeters to 1100 to 1200 C. Radiant heating can be provided in a known manner for preheating the silicon rod until the current absorption through induction becomes possible.

The induction coil for melting the semiconductor rod 2 can be constructed, for example, as a flat coil with two windings. A power input of 2 to 3 kilowatts is sufficient for melting the aforementioned V silicon rod. Naturally the melting coil can also be in the form of a cylindrical coil having a different number of windings.

Supplemental heating with the additional induction coil 7 for a zone-melting process in which numerous passes of the melting zone are made, is carried out only during the final zone pass, as it has been experimentally determined that the vquality of the crystal, for monoc'rystalline growth, for example, depends essentially only on the last zone pass.

It is advantageous, after the zone melting is completed, not to abruptly halt the heating but rather to gradually reduce it. Because the construction of the induction coil 7 is not rotationally symmetrical, incidental stresses arise, and one should therefore proceed as follows after completion of the zone melting. First of all, the semiconductor rod should be severed for example by smelting in the vicinity of the upper clamping location. Then the power to the melting coil 5 should be switched off and thereafter the heat output of the induction coil 7 reduced gradually, for example, over an interval of about 20 minutes, by continuously turning the lower rod-clamping holder 4 and the rod, accordingly, until the silicon rod is cooled from a temperature of about 1100 to 1200 C. to a temperature of about 600 C. Then the power to the induction coil 7 should be switched off.

In FIGS. la and 2 there is shown a semiconductor rod 12 clamped at opposite ends by a pair of clamping holders 13, 14 and heated, for example, by a spirally wound fiat coil 15 to produce a melting zone therein. The heating coil 15 for producing the melting zone can also be a cylindrical coil. The winding portions of another coil 16 extend parallel to the axis of the semiconductor rod 12. The coil 16 is Wound in such a manner that winding portions thereof extend both parallel to one another and to the rod axis. These portions of the windings are disposed on the peripheral surface of an imaginary cylinder whose axis passes through the rod or is coextensive With the rod axis. The parallel portions of the windings are respectively alternately connected to one another at the upper and lower ends thereof, as shown in FIG. 1a. The conductor or conductive line consequently has a meandering or sinuous form so that the current fiow is also over a meandering or sinuous path.

The portions of the windings which extend parallel to one another can be located comparatively close to one another so that a particularly uniform heating of the rod extending parallel thereto can be achieved. As shown in FIGS. 1a and 2, the coil 15, which can be secured for example through its connecting leads to a carriage or slide, can be removed by slipping it upwardly out of the meander-shaped coil which serves for preheating or afterheating the semiconductor rod and which can, if desired, be fixed in position. In a similar manner, by suitably disposing the leads of the coil 15, for example, within the space adjacent to the illustrated space between the parallel portions of the coil 16, shown in FIG. 1a, the coil 15 can be removed by slipping it downwardly out of the meander-shaped conductor line. By the construction of the embodiment shown in FIGS. 1a and 2, the substitution of the induction heating coil 15 which serves for producing the melting zone in the semiconductor rod 12 or a similar operation, is rendered relatively simple. This construction also permits heating in a more uniform manner the rod-shaped semiconductor material that is to be processed.

If desired, the auxiliary coil 6 can be movably mounted so that it can carry out a movement parallel to the movement of the heating coil 15, for example also with the same speed. The additional heating coil wound in meandering form can extend, for example, over a rod length of five to ten centimeters and can be displaced simultaneously with the heating coil 16 which produces a melting zone of five to fifteen millimeters length, along the axis of the rod-shaped semiconductor material. The heating coil 16 can, for example, produce a melting zone of five to fifteen millimeter length and the rod-shaped semiconductor can be 20 to 50 cm. long, for example.

I claim:

1. In an apparatus for crucible-free zone melting of a semiconductor rod, a first induction heating coil for melting a zone of the rod, said coil being located adjacent the rod and displaceable in the direction of the rod axis, and a second induction heating coil for heating the rod to V a temperature below its melting point, said second coil comprising at least one turn having portions extending substantially parallel to each other and to the rod axis.

2. In an apparatus for crucible-free zone melting of a vertically-oriented semiconductor rod, means for clamping the rod at its ends, a first induction heating coil for melting a zone of the rod, said coil being disposed adjacent the rod and movable in the direction of the rod axis, and a second induction heating coil for selectively preheating and after-heating the rod while the rod is in a solid state, said second coil-comprising coil turn portions extending substantially parallel to each other and to the rod axis between said clamping means.

3. In an apparatus for crucible-free zone melting of a vertically-oriented semiconductor rod, stationary clamping means for rigidly clamping the rod at its ends, an induction heating coil for melting a zone of the rod, said coil being disposed adjacent the rod and movable in the direction of the rod axis between said clamping means, and a stationary, subsidiary induction heating coil for selectively preheating and after-heating the rod to a tem perature below its melting point, said subsidiary coil comprising coil turn portions spaced from the rod and extending in a direction substantially parallel to each other and to the rod axis and substantially the entire length of the rod between said clamping means.

4. In an apparatus for crucible-free zone melting of a vertically-oriented semiconductor rod, stationary clamping means for rigidly clamping the rod at its ends, an induction heating coil for melting a zone of the rod, said coil being disposed around and coaxial to the rod and movable in the direction of the rod axis between said clamping means so as to move the melting zone along the rod, and a stationary, subsidiary induction heating coil for selectively pre-heating and after-heating the rod to a temperature below its melting point, said subsidiary coil comprising a plurality of coil turns having portions spaced radially from the rod and said melting coil and extending in a direction substantially parallel to each other and to the rod axis and substantially the entire length of the rod between said clamping means.

5. In an apparatus for crucible-free zone melting of a semiconductor rod, means for clamping the rod at its ends, a first induction heating coil for melting a zone of the rod, said coil being disposed adjacent the rod and mov-. able in the direction of the rod axis, and a second induction heating coil for selectively preheating and after-heat mg the rod while the rod is in a solid state, said second coil comprising coil turn portions extending substantially parallel to each other and to the rod axis between said clamping means, said substantially parallel coil turn portions being disposed in the peripheral surface of an imagi inary cylinder coaxial with the rod and alternately connected with one another at the ends of said imaginary 1cylinder so as to form a continuous meandering conductive 6. Apparatus according to claim 5, said second induction heating coil also being movable in the direction of the rod axis.

7. In an apparatus for crucible-free zone melting of a semiconductor rod, means for clamping the rod at its ends, a first induction heating coil for melting a zone of the rod, said first coil being disposed around and coaxial to the rod and movable in the direction of the rod axis, and a second induction heating coil for selectively preheating and after-heating the rod while the rod is in a solid state, said second coil comprising coil turn portions extending substantially parallel to each other and to the rod axis between said clamping means, said substantially parallel coil turn portions being disposed in the peripheral surface of an imaginary cylinder surrounding and coaxial with the rod and said first coil and alternately connected with one another at the ends of said imaginary cylinder so as to form a continuous meandering conductive line, said first coil having a pair of end leads extending between a pair of adjacent coil turn portions of said second coil whereby said first and second coils are relatively movable apart from one another in the direction of the rod axis and in a direction in which said pair of end leads passes from between said pair of adjacent coil turn portions at an end of said imaginary cylinder at which said adjacent coil turn portions are unconnected.

References Cited by the Examiner UNITED STATES PATENTS NORMAN YUDKOFF, Primary Examiner.

G. HINES, Assistant Examiner. 

1. IN AN APPARATUS FOR CRUCIBLE-FREE ZONE MELTING OF A SEMICONDUCTOR ROD, A FIRST INDUCTION HEATING COIL FOR MELTING A ZONE OF THE ROD, SAID COIL BEING LOCATED ADJACENT THE ROD AND DISPLACEABLE IN THE DIRECTION OF THE ROD AXIS, AND A SECOND INDUCTION IN THE DIRECTION OF THE ROD AXIS, A TEMPERATURE BELOW ITS MELTING POINT, SAID SECOND COIL COMPRISING AT LEAST ONE TURN HAVING PORTIONS EXTENDING SUSBSTANTIALLY PARALLEL TO EACH OTHER AND TO THE ROD AXIS. 